The present invention relates to a color electrophotographic printer having an image bearing body (image bearing belt, image bearing drum, etc.), an intermediate image transfer body (image transfer belt, image transfer drum, etc.) or a medium feeding body (medium feeding belt, medium feeding drum, etc.), and in particular, to a color electrophotographic printer and a feeding speed control method for the color electrophotographic printer by which the feeding speed of the image bearing body, the intermediate image transfer body or the medium feeding body can be controlled appropriately.
In an electrophotographic printer of a standard type, electrical charges which are remaining on its image bearing body (image bearing belt, image bearing drum etc. on which a latent image is formed by means of exposure) since its previous operation are thoroughly erased by exposure to an eraser lamp. Subsequently, the image bearing body is charged uniformly and evenly by means of corona charger or a charging roll. Thereafter, by irradiating the surface of the image bearing body with laser light or LED (Light-Emitting Diode) light, the electrical charge distribution on the surface is altered and thereby a latent image corresponding to a toner image to be generated is formed on the surface. Toner particles, which are attracted to the electrical charge distribution (latent image) on the surface of the image bearing body, forms a pattern corresponding to the electrical charge distribution and thereby the latent image is developed into a toner image.
In color image development, the formation of the latent image and the development are repeated for each color (yellow, magenta, cyan, black). The developed image is transferred to a medium such as paper, a film, etc. directly or via an intermediate image transfer body (image transfer belt, image transfer drum, etc.).
In the case of the color image development, the superposition of the developed color images (yellow, magenta, cyan, black) is conducted on the image bearing body, on the medium such as paper, a film, etc., or on the intermediate image transfer body.
In such color electrophotographic printers, the accuracy of the superposition of the developed color images not only affects the precision of printing position but also considerably affects color reproduction which is attained by the superposition of yellow, magenta, cyan and black. Therefore, in order to obtain high quality printed outputs, the mechanism or control for the color superposition is of critical importance.
Therefore, some attempts have been made in order to conduct the color superposition accurately and implement high print quality. For example, the interval between exposure units (for generating latent images for each color) or the interval between development units (for generating toner images of each color) is managed and set precisely in the color electrophotographic printer, or registration error in the color superposition is actually detected by conducting test printing and the feeding speed of the image bearing body (, the intermediate image transfer body or the medium feeding body) is corrected and/or activation timing of exposure units are adjusted based on the detected registration error.
As for the constant speed feeding control, techniques by use of control systems employing stepping motors, D.C. servomotors, A.C. servomotors, etc. have been established, and thus such constant speed feeding control can be implemented comparatively easily and at a low cost. However, the cylindrical image bearing body (, the cylindrical intermediate image transfer body or the cylindrical medium feeding body), which is driven by the constant speed feeding control, has errors or variations caused by its manufacturing process. Therefore, the accuracy of the color superposition is necessitated to be deteriorated due to the errors even if the constant speed control of motor rotation is conducted precisely. Also when an image bearing body (, an intermediate image transfer body or a medium feeding body) in the shape of a belt is used, a roller for driving the image bearing body (, the intermediate image transfer body or the medium feeding body) has similar errors or variations due to the manufacturing process, thereby the color superposition accuracy is deteriorated in the same way.
In the case where such errors or variations exist in the image bearing body (, the intermediate image transfer body or the medium feeding body), the feeding speed of the image bearing body (, the intermediate image transfer body or the medium feeding body) changes periodically, therefore, the deterioration of the color superposition accuracy (registration error or misregistration in the color superposition) can be avoided by using the periodicity of the feeding speed.
Such techniques using the periodicity of the feeding speed have been disclosed in Japanese Patent Application Laid-Open No.HEI2-156260 (Japanese Patent No.2745599), Japanese Patent Application Laid-Open No.HEI5-103175, etc.
In Japanese Patent Application Laid-Open No.HEI2-156260, four image generation sections corresponding to yellow, magenta, cyan and black (each of which including a dielectric drum, a charging unit, an electrostatic latent image writing head, a development unit and a cleaner) are placed around a cylindrical medium feeding body (which feeds a medium such as paper on which toner images corresponding to the four colors will be superposed) so as to be in the same phase in the feeding speed periodicity. For example, when the feeding speed of the cylindrical medium feeding body changes N times per revolution, the cycle of the feeding speed variation is 360xc2x0 /N and thus the four image generation sections are placed in positions corresponding to Mxc3x97360xc2x0 /N (M: integer) around the cylindrical medium feeding body. By such positioning of the four image generation sections around the cylindrical medium feeding body (medium feeding drum), the registration error in the color superposition is avoided.
In Japanese Patent Application Laid-Open No.HEI5-103175, the exposure position interval on an image bearing belt (that is, the interval between exposure units for the four colors on the image bearing belt) and the diameter of a drive roller for driving the image bearing belt are set so that the exposure position interval will be a multiple of the circumference of the drive roller (Lb=Kxc3x97xcfx80 D (Lb: exposure position interval on the image bearing belt, K: natural number, D: diameter of the drive roller)) for avoiding the misregistration in the color superposition.
However, in the above conventional techniques, the outer diameter of the image bearing drum (, the intermediate image transfer drum or the medium feeding drum) or the outer diameter of the roller for driving the image bearing belt (, the intermediate image transfer belt or the medium feeding belt) is restricted by the exposure position interval or the image transfer position interval (that is, the interval between toner image transfer positions).
The market is requiring miniaturization of the color electrophotographic printers, and thus the internal mechanism and parts of the color electrophotographic printer are also required to be downsized. However, there are limitations in the downsizing of exposure units and development units of the color electrophotographic printers, and thus the reduction of the interval between the exposure units or the development units is also limited.
There is little flexibility in dimensional alteration of the cylindrical image bearing bodies (image bearing drums) since considerable time and cost becomes necessary for constructing production facilities for them. Therefore, there are restrictions both on the interval between the exposure units or the development units and on the outer diameter of the cylindrical image bearing body, therefore, it is difficult to place the exposure units or the development units optimally so as to satisfy a certain relationship as in the conventional techniques.
In the case of the image bearing body in the shape of a belt (image bearing belt), there is flexibility to some extent in the outer diameter of its drive roller, however, from the viewpoint of the endurance of the repeatedly bent image bearing belt on which a photosensitive material is coated, there is also a limit in the downsizing of the drive roller. Therefore, it is also difficult to optimally conduct the miniaturization of the color electrophotographic printer including the image bearing belt satisfying a certain relationship as in the conventional techniques.
It is therefore the primary object of the present invention to provide a color electrophotographic printer and a feeding speed control method for the color electrophotographic printer by which the color superposition accuracy can be improved and thereby high quality printing can be realized without the need of restricting the outer diameter of the image bearing drum, the intermediate image transfer drum or the medium feeding drum or the outer diameter of the roller for driving the image bearing belt, the intermediate image transfer belt or the medium feeding belt.
In accordance with a first aspect of the present invention, there is provided a color electrophotographic printer in which m (m=2, 3, 4, . . . ) latent images corresponding to m colors are successively formed on an image bearing body by m exposure units corresponding to the m colors by means of irradiation of laser light or LED (Light-Emitting Diode) light and development of each latent image corresponding to each color is conducted just after the formation of the latent image and thereby color superposition of the m colors is conducted on the image bearing body. The color electrophotographic printer comprises an encoder pickup wheel and a control section. The encoder pickup wheel is placed in contact with the image bearing body for detecting the feeding speed of the image bearing body. The outer diameter D of the encoder pickup wheel is set as D=L/nxcfx80 (L: interval between the exposure units, n: natural number). The control section controls the feeding speed of the image bearing body by means of closed-loop control based on the feeding speed detected by the encoder pickup wheel.
In accordance with a second aspect of the present invention, in the first aspect, the image bearing body is implemented by an image bearing belt, and the encoder pickup wheel in contact with the image bearing belt detects the feeding speed of the image bearing belt.
In accordance with a third aspect of the present invention, in the first aspect, the image bearing body is implemented by an image bearing drum, and the encoder pickup wheel in contact with the image bearing drum detects the feeding speed of the image bearing drum.
In accordance with a fourth aspect of the present invention, there is provided a color electrophotographic printer having m (m=2, 3, 4, . . . ) exposure units corresponding to m colors and m development units corresponding to the m exposure units each of which including an image bearing roller on which a latent image is formed by exposure by the corresponding exposure unit and a toner image is formed by development by the corresponding development unit, in which the m toner images formed on the m image bearing rollers respectively are transferred to an intermediate image transfer body and thereby color superposition of the m colors is conducted on the intermediate image transfer body. The color electrophotographic printer comprises an encoder pickup wheel and a control section. The encoder pickup wheel is placed in contact with the intermediate image transfer body for detecting the feeding speed of the intermediate image transfer body. The outer diameter D of the encoder pickup wheel is set as D=L/nxcfx80 (L: interval between the development units, n: natural number). The control section controls the feeding speed of the intermediate image transfer body by means of closed-loop control based on the feeding speed detected by the encoder pickup wheel.
In accordance with a fifth aspect of the present invention, in the fourth aspect, the intermediate image transfer body is implemented by an image transfer belt, and the encoder pickup wheel in contact with the image transfer belt detects the feeding speed of the image transfer belt.
In accordance with a sixth aspect of the present invention, in the fourth aspect, the intermediate image transfer body is implemented by an image transfer drum, and the encoder pickup wheel in contact with the image transfer drum detects the feeding speed of the image transfer drum.
In accordance with a seventh aspect of the present invention, there is provided a color electrophotographic printer having m (m=2, 3, 4, . . . ) exposure units corresponding to m colors and m development units corresponding to the m exposure units each of which including an image bearing roller on which a latent image is formed by exposure by the corresponding exposure unit and a toner image is formed by development by the corresponding development unit, in which the m toner images formed on the m image bearing rollers respectively are transferred to a medium which is fed on a medium feeding body and thereby color superposition of the m colors is conducted on the medium which is fed on the medium feeding body. The color electrophotographic printer comprises an encoder pickup wheel and a control section. The encoder pickup wheel is placed in contact with the medium feeding body for detecting the feeding speed of the medium feeding body. The outer diameter D of the encoder pickup wheel is set as D=L/nxcfx80 (L: interval between the development units, n: natural number). The control section controls the feeding speed of the medium feeding body by means of closed-loop control based on the feeding speed detected by the encoder pickup wheel.
In accordance with an eighth aspect of the present invention, in the seventh aspect, the medium feeding body is implemented by a medium feeding belt, and the encoder pickup wheel in contact with the medium feeding belt detects the feeding speed of the medium feeding belt.
In accordance with a ninth aspect of the present invention, in the seventh aspect, the medium feeding body is implemented by a medium feeding drum, and the encoder pickup wheel in contact with the medium feeding drum detects the feeding speed of the medium feeding drum.
In accordance with a tenth aspect of the present invention, there is provided a feeding speed control method for a color electrophotographic printer in which m (m=2, 3, 4, . . . ) latent images corresponding to m colors are successively formed on an image bearing body by m exposure units corresponding to the m colors by means of irradiation of laser light or LED (Light-Emitting Diode) light and development of each latent image corresponding to each color is conducted just after the formation of the latent image and thereby color superposition of the m colors is conducted on the image bearing body. The feeding speed control method comprises a feeding speed detection step and a feeding speed control step. In the feeding speed detection step, the feeding speed of the image bearing body is detected by an encoder pickup wheel which is placed in contact with the image bearing body and whose outer diameter D is set as D=L/nxcfx80 (L: interval between the exposure units, n: natural number). In the feeding speed control step, the feeding speed of the image bearing body is controlled by a control section by means of closed-loop control based on the feeding speed detected in the feeding speed detection step.
In accordance with an eleventh aspect of the present invention, in the tenth aspect, the image bearing body is implemented by an image bearing belt, and the encoder pickup wheel in contact with the image bearing belt detects the feeding speed of the image bearing belt in the feeding speed detection step.
In accordance with a twelfth aspect of the present invention, in the tenth aspect, the image bearing body is implemented by an image bearing drum, and the encoder pickup wheel in contact with the image bearing drum detects the feeding speed of the image bearing drum in the feeding speed detection step.
In accordance with a thirteenth aspect of the present invention, there is provided a feeding speed control method for a color electrophotographic printer having m (m=2, 3, 4, . . . ) exposure units corresponding to m colors and m development units corresponding to the m exposure units each of which including an image bearing roller on which a latent image is formed by exposure by the corresponding exposure unit and a toner image is formed by development by the corresponding development unit, in which the m toner images formed on the m image bearing rollers respectively are transferred to an intermediate image transfer body and thereby color superposition of the m colors is conducted on the intermediate image transfer body. The feeding speed control method comprises a feeding speed detection step and a feeding speed control step. In the feeding speed detection step, the feeding speed of the intermediate image transfer body is detected by an encoder pickup wheel which is placed in contact with the intermediate image transfer body and whose outer diameter D is set as D=L/nxcfx80 (L: interval between the development units, n: natural number). In the feeding speed control step, the feeding speed of the intermediate image transfer body is controlled by a control section by means of closed-loop control based on the feeding speed detected in the feeding speed detection step.
In accordance with a fourteenth aspect of the present invention, in the thirteenth aspect, the intermediate image transfer body is implemented by an image transfer belt, and the encoder pickup wheel in contact with the image transfer belt detects the feeding speed of the image transfer belt in the feeding speed detection step.
In accordance with a fifteenth aspect of the present invention, in the thirteenth aspect, the intermediate image transfer body is implemented by an image transfer drum and the encoder pickup wheel in contact with the image transfer drum detects the feeding speed of the image transfer drum in the feeding speed detection step.
In accordance with a sixteenth aspect of the present invention, there is provided a feeding speed control method for a color electrophotographic printer having m (m=2, 3, 4, . . . ) exposure units corresponding to m colors and m development units corresponding to the m exposure units each of which including an image bearing roller on which a latent image is formed by exposure by the corresponding exposure unit and a toner image is formed by development by the corresponding development unit, in which the m toner images formed on the m image bearing rollers respectively are transferred to a medium which is fed on a medium feeding body and thereby color superposition of the m colors is conducted on the medium which is fed on the medium feeding body. The feeding speed control method comprises a feeding speed detection step and a feeding speed control step. In the feeding speed detection step, the feeding speed of the medium feeding body is detected by an encoder pickup wheel which is placed in contact with the medium feeding body and whose outer diameter D is set as D=L/nxcfx80 (L: interval between the development units, n: natural number). In the feeding speed control step, the feeding speed of the medium feeding body is controlled by a control section by means of closed-loop control based on the feeding speed detected in the feeding speed detection step.
In accordance with a seventeenth aspect of the present invention, in the sixteenth aspect, the medium feeding body is implemented by a medium feeding belt, and the encoder pickup wheel in contact with the medium feeding belt detects the feeding speed of the medium feeding belt in the feeding speed detection step.
In accordance with an eighteenth aspect of the present invention, in the sixteenth aspect, the medium feeding body is implemented by a medium feeding drum, and the encoder pickup wheel in contact with the medium feeding drum detects the feeding speed of the medium feeding drum in the feeding speed detection step.